Systems and methods for actuator power failure response

ABSTRACT

A brushless DC motor is controlled by determining a speed of the motor and a line voltage supplying drivers of the motor. Based upon these determined values and predetermined motor performance data, a maximum PWM on-time is determined for drive signals supplied to windings of the motor. This maximum PWM on-time thereby limits motor torque output to a predetermined maximum. Stopping the motor is performed in sequential phases including a loaded generator phase, an active braking phase and a holding phase. During holding, the amount of energy supplied to the motor is reduced to the point just before slippage such that holding energy is minimized. Upon power failure, the motor is driven toward a spring return home position overcoming static friction that may prevent the spring return mechanism from functioning properly. These techniques are applied to a motor in an actuator.

CROSS REFERENCE TO RELATED APPLICATIONS

This Patent application is related to copending U.S. patentapplications, "Systems and Methods for Braking of Actuator and BrushlessDC Motor Therein," Atty. Dkt. No. 97,308, Ser. No. 08/920,053, filed onAug, 28, 1997, and "Systems and Methods for Torque Control of Actuatorand Brushless DC Motor Therein," Atty. Dkt. No. 97,307, Ser. No.08/920,052, filed on Aug. 28, 1997.

TECHNICAL FIELD

The present invention relates, in general, to motor control technology.More specifically, the present invention relates to methods and systemsfor controlling brushless DC motors.

BACKGROUND OF THE INVENTION

Brushless DC motors represent an attractive motor technology for manyapplications such as, for example, actuators. Advantageously, they havehigh operating efficiencies and high power densities. However,controlling these brushless DC motors is relatively complex, requiringdedicated controllers and multiple high-power semiconductor drivers.

One aspect of brushless DC motor control is regulating output torque.This is conventionally performed using motor current sensing. Tosummarize this technique, one or more current sensing resistors areplaced in series with one or more of the motor windings. The voltagedrop across the current sensing resistor(s) is measured, and indicatesmotor torque. Unfortunately, the use of current sensing resistors addsexpense in terms of parts count, space required, cooling requirements,and energy efficiency.

A further complication of using brushless DC motors relates to startingand stopping them, particularly when under load. One previously usedmotor stopping technique involved maintaining high power on a particularwinding combination to `lock` the motor in position. However, thisconsumes large amounts of power, generates excess heat, and is oftenineffective if slippage occurs.

The above-described problems are multiplied when a brushless DC motor isincluded in another device such as, for example, an actuator. Static anddynamic loads resulting from the actuator application are imparted onthe motor further complicating control thereof.

The present invention is directed toward solutions to theabove-identified problems.

SUMMARY OF THE INVENTION

Briefly described, in a first aspect, the present invention includes amethod for use in controlling a brushless DC motor. The motor isenergized by multiple drivers, wherein the drivers supply energy to themotor based upon a motor winding commutation pattern and a PWM controlsignal used therewith. The speed of the motor is thereby controlled.

In particular, the method includes determining the speed of the motorand a line voltage supplying the drivers of the motor. Then, based uponthe speed, the line voltage and predetermined motor performance data, amaximum PWM on-time for the PWM control signal is determined. Themaximum PWM on-time corresponds to a predetermined motor torque output.The motor performance data may include constant torque curves forcombinations of line voltage, motor speed and PWM on-time.

As an enhancement, the motor may include at least one positional sensorused in determining the time between positional changes. The positionalinformation may be accumulated to facilitate determining absolute motorposition in the form of at least a number of motor rotations.

As a further enhancement, the motor may be included within an actuatorand coupled to an output member thereof through reduction gearing. Inthis configuration, the method may include tracking a position of theoutput member of the actuator using the determined absolute motorposition. The method may also include controlling rotation of the motorto selectively position the output member of the actuator. During thepositioning of the actuator, the speed of the motor is regulated usingthe PWM control signal. More particularly, the PWM control signal may belimited to the maximum PWM on-time to limit motor torque output to thepredetermined motor torque output.

In another aspect, the present invention includes a method of operatingan actuator that includes an output and a brushless DC motormechanically coupled thereto. The brushless DC motor is controlled atleast in part by a PWM control signal operable to control an amount ofenergy transferred to the motor.

More particularly, the method comprises positioning the actuator byoperating the motor. During the positioning, output torque of the motoris controlled (the controlling includes collecting operational motordata that is exclusive of motor current consumption). This data is usedin combination with predetermined motor performance data to regulatemotor torque using the PWM control signal.

In a further aspect, the present invention includes a method forstopping rotation of a brushless DC motor. The method includes operatingthe motor as a loaded generator. During this loaded generator operation,a motion profile of the motor is monitored.

To continue, when the motion profile of the motor reaches apredetermined characteristic, an active deceleration mode is engaged.This mode operates by driving the motor in a direction opposite to itsrotation. When a reversal of motor rotation occurs, a holding mode isengaged to hold the motor position static.

As an enhancement to the above, the motor may include multiple windings,and operating the motor as a loaded generator may include grounding atleast one of the windings. Also, monitoring the motion profile of themotor may include monitoring at least one of a speed, a decelerationprofile, and a number of rotations of the motor.

As a further enhancement, wherein the motor includes multiple windings,the active deceleration mode may includes monitoring a rotationalposition of the motor and a direction of rotation of the motor. Inresponse to the monitored position and direction, a pattern of windingsis engaged that is rotationally behind a current motor position. Thisprovides energy to decelerate the motor. This pattern may be updated asthe motor continues to rotate. Further, the holding mode includes theengagement of a pattern on the windings to hold the motor steady.

In yet another aspect, the present invention includes a method forholding a brushless DC motor steady while minimizing an amount of energyengaged on the windings. The method includes setting an on-time of thePWM control signal to a first predetermined value. A motor windingcommutation pattern is applied to the motor to hold the motor in itscurrent position. The on-time of the PWM control signal is thendecreased and a check for motor slippage is performed. If motor slippageoccurs, the on-time of the PWM control signal is increased until theslippage stops.

As an enhancement, the first predetermined value may comprise a PWMon-time previously used to stop rotation of the motor. Further, when theslippage occurs, the motor commutation pattern is updated to correspondto a current, slipped, motor position.

As a further enhancement, the motor may be included in an actuator whereit mechanically drives an output member thereof. The method therebyincludes holding the output member of the actuator in a designatedposition. Also, the method may include executing a positional controlfeedback loop to selectively operate the motor to place the outputmember of the actuator in a selected position, and hold it in theselected position. The positional control feedback loop may includehysteresis about its positional set-point. Thus, repositioning of themotor is not performed unless the slippage of the motor exceeds apredetermined amount.

In an additional aspect, the present invention comprises a method ofoperating an actuator that includes an output member and a brushless DCmotor mechanically coupled thereto. The output member has a springreturn mechanism coupled to it that provides bias toward a homeposition. The method includes detecting a power failure of powersupplied to the actuator. Upon power failure, the brushless DC motor isdriven to move the output member of the actuator toward the homeposition. This overcomes static friction that may prevent the springreturn mechanism from working. Further, a check may be performed toinsure that the actuator is not already in home position beforebeginning motion thereto.

The actuator may include a power supply, and as an enhancement, drivingthe brushless DC motor may use residual energy stored in the powersupply. Further detecting the power failure may include monitoring apower supply voltage within the actuator and sensing a power failurewhen the power supply voltage falls below a predetermined level. Themonitored power supply may comprise a power supply voltage for driversof the brushless DC motor.

In further aspects, the present invention includes apparatuscorresponding to the above-described methods.

The present invention has several advantages and features associatedwith it. Torque control of a brushless DC motor is performed without theuse of current sensing techniques. This facilitates the elimination of,for example, sensing resistors that consume energy, occupy space anddissipate heat. Also, actuator stopping and holding is performed in amanner that minimizes the amount of energy required. Further, problemsassociated with motor cogging and other static friction interfering withthe spring return mechanism are mitigated. Thus, the techniquesdisclosed herein advance the art of actuator and motor control.

BRIEF DESCRIPTION OF THE DRAWINGS

The subject matter regarded as the present invention is particularlypointed out and distinctly claimed in the concluding portion of thespecification. The invention, however, both as to organization andmethod of practice, together with further objects and advantagesthereof, may best be understood by reference to the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of an actuator pursuant to an embodiment ofthe present invention;

FIG. 2 is a block diagram of the electronics of the actuator of FIG. 1in accordance with one embodiment of the present invention;

FIGS. 3-4 are example graphs of motor performance data in conformancewith an embodiment of the present invention;

FIG. 5 is a flow-diagram of actuator operation pursuant to oneembodiment of the present invention;

FIG. 6 is a flow-diagram of a technique for actuator response topositional commands in accordance with an embodiment of the presentinvention;

FIG. 7 is a flow-diagram of a technique for commuting a brushless DCmotor in response to an actuator position set-point change pursuant toone embodiment of the present invention;

FIG. 8 is a flow-diagram of a torque control technique in accordancewith an embodiment of the present invention;

FIG. 9 is a flow-diagram of a technique for stopping actuator/motormotion in conformance with one embodiment of the present invention;

FIG. 10 is a flow-diagram of a technique for holding a brushless DCmotor/actuator in a static position in conformity with one embodiment ofthe present invention; and

FIG. 11 is a flow-diagram of a technique for responding to a power losscondition according to one embodiment of the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Beginning with FIG. 1, a block diagram of an actuator 10 is depicted. Ingeneral, actuator 10 may be used to position a wide variety ofmechanisms attached thereto. In one example, the actuator may beattached to a ventilation louver assembly. In another example, theactuator may be attached to a valve. More particularly, and by way ofexample, the actuator described herein has a rotational output member(e.g., a shaft). However, this output member may be mechanicallytranslated to provide other output members such as, for example, alinear output member.

Turning to the embodiment of FIG. 1, actuator 10 includes controlelectronics 11 which receive power and control inputs. Controlelectronics 11 are coupled to a motor 13 to both energize the motor andto receive a positional feedback signal 14 therefrom. Motor 13 ismechanically coupled to an output shaft 19 vis-a-vis gearing 15. Thegearing 15 has a spring return mechanism 17 coupled to it such that uponmotor deenergization, output shaft 19 will be biased toward a homeposition.

In the current embodiment, gearing 15 is reduction gearing with a large(e.g., 14,000:1) reduction ratio. This facilitates an applicationrequiring relatively high output torque at a relatively low speed.However, this gearing can be modified depending on the particularactuator application requirements. Moreover, spring return 17 may beeliminated if a mechanical home return mechanism is not desired.

Turning to FIG. 2, the electronics 11 and motor 13 aspects of actuator10 are more particularly depicted. A processor 21 provides control ofactuator 10. Processor 21 is selected based upon applicationrequirements (e.g., memory, power consumption, computing power, I/O,etc.) and in one embodiment, a Philips Semiconductors brand, model83C749 CMOS single-chip 8-bit microcontroller is used.

Processor 21 is coupled to motor 13 through three half-bridge powerswitches 43, 45 and 47. Each switch is a driver for a different winding(i.e., phase) of motor 13 which is a brushless DC motor. Embodimentswith other than three windings would have a corresponding number ofdrivers. The top half of each switch is driven from processor 21 outputsA_(T), B_(T) and C_(T). The bottom half of each switch is driven bysignals from processor 21 outputs A_(B), B_(B), and C_(B) as ANDed witha PWM output of processor 21 by gates 41, 39 and 37 (either discretegates, or implemented in programmable logic or software). The PWMcontrol signal is used to control the amount of energy transferred tomotor 13 and will be explained in further detail hereinbelow.

The assembly of motor 13 includes position sensors 48A, 48B and 48Cwhich in the current embodiment are hall-effect sensors. These sensorsprovide a feedback signal to processor 21 such that processor 21 maydetermine the current rotational position of motor 13 (within the limitsof the sensor configuration). As is well known in regard to brushless DCmotors, this positional information may be generally used to, forexample, determine winding engagement patterns for controlling motorrotation. Other positional sensors may be used in other embodiments andinclude, for example, optical sensors and back-EMF sensing techniques.

Power for actuator 10 is provided by a power supply 23 that has anoutput, V_(MOT), for powering the motor drivers (43, 45 & 47) and anoutput, V_(LOGIC), for powering electronics including the digital logicwithin actuator 10.

Processor 21 has several analog inputs used according to the presentinvention. To note, in the current embodiment, the analog inputs areintegral with the processor 21 and are multiplexed through a common A/Dconverter; however, other A/D topologies can be used according to thetechniques described herein.

A first analog input, A/D(1) reads the voltage at a node within avoltage divider 25 connected between the motor supply, V_(MOT), and acommon. This input is used to determine V_(MOT) for use with the motorcontrol techniques described hereinbelow. A second analog input A/D(2)is connected to a "speed select" potentiometer 27 that a user adjusts tocontrol actuator speed. A third analog input A/D(3) is connected to a"stroke select" potentiometer 29 that a user adjusts to control actuatorstroke (e.g., degrees of rotation or length of travel). In the currentembodiment, potentiometers are used for the above-described user input;however, other forms of user input are possible such as, e.g., through acomputer interface, or though a user I/O keypad/display facility. Also,if adjustment is unnecessary in a particular embodiment, the adjustmentscould be eliminated in favor of preprogrammed settings.

A fourth analog input A/D(4) receives a control signal for proportionalcontrol of actuator position. Conditioning electronics 31 provideadjustment of the control signal voltage/current to be compatible withthe A/D(4) input of processor 21. As one example, if the control inputhas a range of 0-10V, electronics 31 may reduce the input voltage byone-half to result in a 0-5V range. Further, electronics 31 provide zeroand span adjustment for the control signal. Zero and span adjustmentcould be performed digitally in an alternate embodiment.

Two digital inputs, D(1) and D(2), are connected to processor 21 andprovide actuator control signals alternate to the proportional controlinput described above. An "open" and a "closed" signal are provided toD(1) and D(2), respectively, and will cause the actuator output toappropriately move in one of an open and a closed direction whenengaged.

Processor 21 is connected to an external serial interface throughcommunications drivers 33 (e.g., RS-232, 485, etc.). This serialinterface has many uses including computer control/feedback of actuatorposition and configuration parameters, test and diagnostics, calibrationand software updating.

Further, a variety of outputs can be added to the circuit of FIG. 2 toprovide indications of position. For example, a digital output canindicate a positional set-point being attained. Also, an analog outputcan be added to provide a voltage or current output related to actuatorposition within its stroke.

Lastly concerning FIG. 2, a non-volatile memory 35 (e.g., FEPROM) isconnected to processor 21 and is used to store configuration informationfor actuator 10. For example, non-volatile memory 35 may contain motorcharacterization data for use as described herein. Alternately, ifsufficient memory of the appropriate type exists in processor 21 itself,it could be used instead of an external memory device such as, forexample, non-volatile memory 35. The particular memorymodel/configuration used will vary as a function of design andapplication requirements.

During actuator operation, the PWM on-time of the motor control signalis limited to a dynamically computed maximum value such that maximumactuator torque output is limited. Various factors influence the maximumPWM on-time such as motor driver supply voltage, motor speed andpredetermined motor performance data.

In one embodiment, example graphs of motor performance data organizedaccording to the present invention are depicted in FIGS. 3-4. RegardingFIG. 3, a constant torque curve 61 is determined for motor 13 using adynamometer. The torque used is the maximum actuator torque. This graphdata is shown in connection with a normalized PWM value at the lowestintended motor driver supply voltage (e.g., 22 volts). Each segmentshown corresponds to approximately 7.5% of a period of rotation (T) atthe design operating speed. Thus a particular segment number n on thegraph corresponds to a rotational period of (1+0.075 n)T, which is ameasure of the speed of the motor.

Turning to FIG. 4, a graph of a normalized compensation factor versusmotor voltage is shown (trace 63). In operation, as motor voltageincreases, this compensation value is used to reduce the PWM on-time tomaintain the predetermined torque limit.

Thus, to summarize, the motor speed and driver voltage are dynamicallymonitored during motor operation. As they vary, a maximum PWM on-time isappropriately updated, and corresponds to a maximum desired outputtorque. For example, since the torque of a DC motor increases as speeddecreases, the maximum PWM on-time will be decreased as motor speeddecreases (thereby limiting torque output). Also, since torque increaseswith motor voltage, as the driver voltage increases, the maximum PWMon-time is decreased (thereby limiting torque output). The determinedmaximum PWM on-time is used as a limit on the actual PWM control signalapplied to the motor drivers (through logic circuitry described herein).

The above control scheme executes continuously during actuator operationsuch that the maximum torque limit is rigorously enforced. In oneembodiment, this torque limiting technique is implemented in connectionwith a "watchdog" timer set to expire at about two times the normal timebetween hall code changes at desired operating speed (e.g., 1 ms). Astall is detected when the motor is operating at the predeterminedtorque limit and motor speed is less than 1/4 of desired operating speed(and the motor is not initiating rotation from being stopped).

Many different implementations of the above technique are possible.However, in a preferred embodiment, the maximum PWM on-time (i.e., a PWMlimit) is dynamically determined as:

    Max.sub.-- pwm.sub.-- on-time=(Model.sub.-- Factor)(Speed.sub.-- Factor)(Voltage.sub.-- Factor)

Model₋₋ Factor is a preset value that defines the desired torque outputfor the specific actuator model. It is the PWM on-time that correspondsto the desired torque output at the desired operating speed and thelowest operating voltage (e.g., one embodiment is designed for 1500 RPM,22 volt minimum, 1.3 oz.-in. torque).

Speed₋₋ Factor is a normalized representation of the PWM on-timerequired to generate a specific (the desired) output torque at aspecific speed (i.e., a specific time between hail changes) at thelowest operating voltage. Speed (period) is the independent variable(measured) and PWM on-time is the calculated value (dependent). Speed₋₋Factor is maximum at the desired operating speed, and decreases andspeed is decreased (period increases). Speed₋₋ Factor is determined frommotor performance data such as, for example, that of FIG. 3.

Voltage₋₋ Factor is a normalized representation of the motor's operatingvoltage. It has a maximum value (1) at the lowest specified operatingvoltage and decreases as operating voltage increases. Voltage Factor isdetermined from motor performance data such as, for example, that ofFIG. 4.

In the current embodiment, the measurement of motor rotational speed isthe time between positional changes of the motor (e.g., time betweenhall sensor counts). To note, this measure is inverse of, e.g., aRPM-type rotational speed measurement. In the current embodiment, allcomputations are based upon the time between positional changes forcomputational efficiency. However, the techniques of the presentinvention are usable with a variety of motor speed measurements.

Operational sequences are now depicted in accordance with the presentinvention. Turning first to FIG. 5, a high-level actuator controlsequence is depicted. Initialization is first performed (STEP 101) andincludes, for example, basic processor booting, memory integrity checks,and software initialization including, for example, variable allocation.Next, the home position of the actuator is determined (STEP 103). Thisis performed by moving the actuator toward home position until themechanical stop of the system is detected. The mechanical stop isdetected by monitoring the motor speed upon torque limit (see, forexample, FIG. 8 hereinbelow). When motor stall limits are reached (i.e.,torque limit is reached at less than 1/4 desired operating speed), it isassumed that the mechanical limit of the actuator is reached.Accordingly, once this travel limit is detected, by using the knowngearing ratios in combination with monitoring motor rotation, the exactactuator position is always known. At additional cost, additionalsensors could be added to detect actuator position (e.g., an opticalencoder or potentiometer)

As a next step, several tasks are maintained active during actuatoroperation. Firstly, a task is executed that responds to user requestsfor actuator operations (STEP 105). Secondly, a task is executed thatactively maintains the set point (desired position) for the actuator(STEP 107). Thirdly, the power supply for the actuator is monitored suchthat appropriate action may be taken upon power failure (STEP 109).These tasks are described in further detail hereinbelow.

Depicted in FIG. 6 is a flow-diagram of a technique for actuatorresponse to user requests. To begin, a user command is received (STEP121). In the current embodiment, this command may arrive through theanalog (proportional position signal), digital (OPEN and CLOSE signals),and computer (RS-232/485/422, etc.) interfaces. Depending on theparticular actuator command, one of three procedures may be initiated(STEP 125). If an actuator open command is received, a procedure iscalled to move the actuator toward an open position (STEP 123). If anactuator close command is received, a procedure is called to move theactuator toward its closed position (STEP 129). If a command is receivedthat calls for the actuator to move to a particular position (e.g.,through the proportional analog input or through the computerinterface), a procedure is called to move the actuator thereto (STEP127).

In the current embodiment, the procedure used to move the actuator to aposition operates using a desired absolute motor position. Absolutemotor position is determined from the user specified actuator positionin combination with the known gearing ratios between the motor andactuator output. Absolute motor position, as used herein, is defined asthe rotational position of the motor, in terms of revolutions,throughout complete actuator travel. Thus, in the current embodiment,wherein the actuator has a maximum output rotation of 180 degrees, andthe gearing ratio to the motor is 14,000:1, absolute motor positionranges from 0 to 7,000 rotations (fractional values permitted).Rotational position, as used herein, refers to the rotational positionof the motor within the sensing limits of the positional sensors.

The operation of the procedure used to move the motor to a specificabsolute position is depicted in FIG. 7. To begin with referencethereto, a direction of rotation is determined by comparing the desiredabsolute motor position to the current absolute motor position (STEP141). Current rotational motor position is next determined from themotor's positional sensors (STEP 143). Then, based upon the currentrotational position, and the direction of rotation, an appropriatewinding pattern is determined and applied to the motor drivers (STEP145).

As an example, if clockwise rotation is desired, the motor windingpattern that will elicit clockwise rotation from the motor's currentrotational position is engaged upon the motor's windings. In the currentembodiment, as one implementation example, the winding pattern update isperformed every 555 microseconds at the design operating speed. Thewinding pattern update is performed fast enough such that it does notprimarily govern motor speed, which is primarily regulated by the PWMcontrol signal on-time described hereinbelow in connection with FIG. 8.

To continue, a counter used to track absolute motor position isincremented or decremented (depending on rotational direction) tomaintain absolute motor position (STEP 147). In the current embodiment,absolute motor position is maintained in units of hall counts, that is,hall-code changes. Thus, if a hall-code change did not occur betweeniteration of the main loop of FIG. 7, the INC/DEC of STEP 147 is notperformed.

A test is next performed to determined if the desired absolute motorposition has been reached (STEP 151). If the desired absolute motorposition has not been reached, the loop iterates to STEP 143. If thedesired absolute motor position has been reached, a procedure isexecuted which will stop and hold the motor (STEP 153). The motor willalso be stopped if a motion command is released, such as, from the OPENor CLOSE digital inputs of the actuator. When the command is released,the current positional set point for the actuator is updated to reflectthe current position.

During operation of the actuator as described above, a torque limitingprocedure is executed to maintain maximum actuator torque below apredetermined limit. This control is performed as part of the motorspeed control according to the technique depicted in the flow-diagram ofFIG. 8. To recall, desired motor speed is set as a user input parameterusing, for example, the potentiometric or computer interface inputdepicted in FIG. 2.

To begin regarding FIG. 8, a motor speed measurement is taken (e.g.,determine the time between hall counts--STEP 161). The motor drivervoltage is also determined (STEP 163). Using the motor speed measurementand the motor driver voltage, a maximum PWM on-time is determined fromthe motor performance data of, for example, FIGS. 3-4 (STEP 165). Thismaximum PWM on-time will set an upper torque limit for the motor in itscurrent operating state.

Continuing, a desired speed for the motor is compared to the actualmotor speed (again, in the current embodiment, speed calculations areperformed with respect to the time between positional motorchanges--STEP 167). If the actual speed is less than the desired speed,the PWM on-time is increased (STEP 173). This will cause more energy tobe transferred to the motor windings thereby increasing its speed. Thenew PWM on-time is compared to the maximum PWM on-time used as a torquelimit (STEP 175), and if the PWM on-time exceeds the maximum, it islimited thereto (STEP 177). The technique then iterates with the readingof the motor speed at STEP 161.

If, upon comparison of actual speed to desired speed (STEP 167, 169),the actual speed exceeds the desired speed, then the PWM on-time isdecreased (STEP 171). The technique then continues with the reading ofthe motor speed at STEP 161. Thus, motor speed and torque control havebeen performed.

Momentarily turning back to FIG. 7, when the absolute motor position hasreached its desired set-point and stopping/holding of the motor isdesired (STEP 153), the techniques depicted in FIG. 9 are instantiated.The below described techniques stop and hold the motor/actuator whileadvantageously minimizing power requirements therefor.

Stopping the motor/actuator motion begins with operating the motor as aloaded generator (STEP 191). This is performed by grounding each of themotor windings through their corresponding drivers (by engaging thelower halves of the half-bridge drivers).

The loaded generator braking mode is maintained, while monitoring themotor's motion profile (e.g., speed, deceleration, and/or rotations)(STEP 193). This braking mode becomes less effective as the motor slows.Thus, in one embodiment, loaded generator braking is held until motorspeed is reduced by 1/2 or 8 mechanical revolutions occur (whicheveroccurs first).

The succeeding braking mode is an active deceleration mode including aloop that begins within a determination of rotational direction (STEP195). Further, the current motor position is determined (STEP 197). Atest is next performed to determine if the rotational direction hasreversed within the active deceleration loop (STEP 197).

If rotational direction has not reversed, active deceleration ismaintained by engaging the motor windings in a pattern designed toachieve reversal of rotational direction (STEP 201). The particularpattern will vary with the motor parameters. In the current embodiment,the determined motor position and direction of rotation are used indetermining and engaging (through a look-up table) a winding patternrotationally "behind" the motor position. This engaged pattern willprovide force in a direction opposite of the current motor rotation.Eventually, the motor will slow, stop and then reverse direction. Untilreversal of direction has occurred, the loop iterates (to STEP 195).

A reversal of direction indicates that deceleration has completed andthe motor should then be held static as described below. As the activedeceleration loop has a time resolution of a fraction of a motorrotation, the initial reversal of direction results in very littlerotational momentum, and thus, active braking in the again reversedirection is not required.

To reiterate, when rotation has reversed (STEP 197), a holding techniquebegins. First, given the current motor position, a winding pattern isengaged to enhance the natural cogging of the motor about its currentrotational position (STEP 203). Rotational stability is awaited for ashort time (e.g., 1/2 second) and the winding pattern is updated if somerotational slippage occurs (STEP 205). Accordingly, the motor is stoppedand held in position. Next, techniques are practiced to maintain themotor's held position, and to minimize the energy required therefor(STEP 207).

During the above-discussed active braking technique, the PWM on-time isramped up to facilitate braking. The PWM on-time begins at apredetermined minimum and is ramped up during active braking until alimit is reached. The predetermined minimum, limit and ramp rate willvary based upon motor, actuator and application requirements; however,in one embodiment, the predetermined minimum is 5% of the Model₋₋Factor, the limit is 25% of Model₋₋ Factor and the ramp rate is 1 countper 1/4 mechanical revolution. Each count is equal to 1/256 of the PWMon time range (0-255) and is thus equal to a change of approximately0.4%.

Turning to FIG. 10, the motor/actuator holding technique begins withmaintaining the currently engaged holding pattern on the motor'swindings (STEP 221). However, according to the techniques disclosedherein, the energy applied to the motor to hold it in place is nowreduced until a minimum of holding energy is achieved. Accordingly, aloop in entered wherein the PWM on-time used for the holding windingpattern is gradually stepped down (STEP 223) (e.g., at a rate of 1 countper 1/4 second) until either slippage (STEP 225), or zero PWM on-time isachieved (STEP 226).

If slippage occurs, the holding pattern on the motor windings is updatedif required to hold the motor in its current (slipped) position (STEP227). A loop is then entered wherein the PWM on-time of the motorwindings is stepped-up (STEP 229) until rotation ceases (STEP 233).During this loop, the winding pattern is updated as required to the holdrotation in the current motor position (STEP 231). Once rotation hasceased, the current winding pattern and PWM on-time are maintained andthe holding routine ends (STEP 235). In one embodiment, if slippageoccurs and the PWM on-time is at its upper limit for active braking, theactive braking loop is reentered.

Turning back to FIG. 5, during operation of the actuator, the positionalset point thereof (i.e., corresponding to absolute motor position) ismonitored for change (STEP 107). Should deviation from the set pointexceed a predetermined amount, repositioning of the actuator at thedesired set-point is performed. Varying degrees of hysteresis can beincorporated into this control scheme to accommodate user requirements.

Further regarding FIG. 5, the power supply voltage is monitored suchthat its falling below a certain level is detected thereby indicating apower failure (STEP 109). In the current embodiment, the motor supplyvoltage is monitored for this purpose as it is conveniently alsomonitored for motor control purposes.

Upon a power failure, absent any intervening techniques, the motorwindings are deenergized and the motor is free to turn under theactuator load. This load can include any loads on the devicemechanically coupled to the actuator and a load from the optional springreturn mechanism in the actuator. The spring return is designed toreturn the actuator to a home position upon a power failure event.However, the natural cogging of the brushless DC motor used incombination with high gearing ratios may prevent the spring return fromoperating properly. The cogging of the motor may provide sufficientstatic friction in the actuator system to prevent the spring returnmechanism from operating. Thus, according to the present invention, atechnique for overcoming the static friction of the actuator system(including that resulting from motor cogging and other sources) uponpower failure is depicted in FIG. 11.

To summarize, upon power failure, the remaining capacitively storedenergy in the actuator power supply is used to begin actuator motion inthe homing direction of the spring return. This overcomes the staticfriction of the system, and once power has completely dissipated, thedynamics of the spring return continue the actuator motion to the homeposition.

In particular, after power failure is detected, a check is performed todetermine whether actuator homing upon power failure is desired (STEP251). If homing is not desired, the routine stops (STEP 253). Forexample, if no spring return mechanism is used, actuator homing uponpower failure will not be active.

If actuator homing upon power failure is desired, a test is nextperformed to detect if the actuator is already in its home position(STEP 255). If so, the routine does not continue (STEP 257). If theactuator is not in home position, a routine is called (e.g., FIG. 7) tomove the actuator to its home position (STEP 259). As motion begins, thestatic friction of the motor cogging is overcome, and once power hascompletely dissipated, the dynamics of the spring return continue theactuator motion to the home position.

The present invention has several advantages and features associatedwith it. Torque control of a brushless DC motor is performed without theuse of current sensing techniques. This facilitates the elimination of,for example, sensing resistors that consume energy, occupy space anddissipate heat. Also, actuator stopping and holding is performed in amanner that minimizes the amount of energy required. Further, problemsassociated with motor cogging and other static friction interfering withthe spring return mechanism are mitigated. Thus, the techniquesdisclosed herein advance the art of actuator arid motor control.

While the invention has been described in detail herein, in accordancewith certain preferred embodiments thereof, many modifications andchanges thereto may be affected by those skilled in the art.Accordingly, it is intended by the appended claims to cover all suchmodifications and changes as fall within the true spirit and scope ofthe invention.

What is claimed is:
 1. A method of operating an actuator including anoutput member and a brushless DC motor mechanically coupled thereto,said output member having a spring return mechanism coupled to it, saidspring return mechanism biasing said output member toward a homeposition thereof, said method comprising:detecting a power failure ofpower supplied to said actuator; upon said power failure driving saidbrushless DC motor to move said output member of said actuator towardsaid home position.
 2. The method of claim 1, wherein said actuatorincludes a power supply, and wherein said driving said brushless DCmotor uses residual energy stored in said power supply of said actuator.3. The method of claim 1, wherein said detecting a power failureincludes monitoring a power supply voltage within said actuator, a powerfailure being detected when said power supply voltage falls below apredetermined level.
 4. The method of claim 3, wherein said power supplyvoltage comprises a power supply voltage for drivers of said brushlessDC motor.
 5. A method of operating an actuator including an outputmember and a brushless DC motor mechanically coupled thereto, saidoutput member having a spring return mechanism coupled to it, saidspring return mechanism biasing said output member toward a homeposition thereof, said method comprising:detecting a power failure ofpower supplied to said actuator; upon said power failure driving saidbrushless DC motor to move said output member of said actuator towardsaid home position if said output member is not at said home position.6. A method of operating an actuator including a output member and abrushless DC motor mechanically coupled thereto, said output memberhaving a spring return mechanism coupled to it, said spring returnmechanism biasing said output member toward a home position thereof,said method comprising:monitoring a power supply voltage of saidactuator; detecting a power failure when said power supply voltage fallsbelow a predetermined voltage level; upon said power failure drivingsaid brushless DC motor to move said output member of said actuatortoward said home position; and wherein said driving said motor overcomesstatic friction.
 7. An apparatus for operating an actuator including anoutput member and a brushless DC motor mechanically coupled thereto,said output member having a spring return mechanism coupled to it, saidspring return mechanism biasing said output member toward a homeposition thereof, said apparatus comprising:means for detecting a powerfailure of power supplied to said actuator; means for, upon said powerfailure, driving said brushless DC motor to move said output member ofsaid actuator toward said home position.
 8. The apparatus of claim 7,wherein said actuator includes a power supply, and wherein said meansfor driving said brushless DC motor uses residual energy stored in saidpower supply of said actuator.
 9. The apparatus of claim 7, wherein saidmeans for detecting a power failure includes means for monitoring apower supply voltage within said actuator, a power failure beingdetected when said power supply voltage falls below a predeterminedlevel.
 10. The apparatus of claim 9, wherein said power supply voltagecomprises a power supply voltage for drivers of said brushless DC motor.11. An actuator comprising:an output member; a brushless DC motormechanically coupled to said output member; a spring return mechanismcoupled to said output member and biasing said output member toward ahome position; means for detecting a power failure of power supplied tosaid actuator; means for, upon said power failure, driving saidbrushless DC motor to move said output member of said actuator towardsaid home position if said output member is not at said home position.